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Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques

Abstract

Magnetic skyrmions are stabilized by a combination of external magnetic fields, stray field energies, higher-order exchange interactions and the Dzyaloshinskii–Moriya interaction (DMI)1,2,3,4,5,6. The last favours homochiral skyrmions, whose motion is driven by spin–orbit torques and is deterministic, which makes systems with a large DMI relevant for applications. Asymmetric multilayers of non-magnetic heavy metals with strong spin–orbit interactions and transition-metal ferromagnetic layers provide a large and tunable DMI4,5,6,7,8. Also, the non-magnetic heavy metal layer can inject a vertical spin current with transverse spin polarization into the ferromagnetic layer via the spin Hall effect9. This leads to torques10 that can be used to switch the magnetization completely in out-of-plane magnetized ferromagnetic elements, but the switching is deterministic only in the presence of a symmetry-breaking in-plane field11,12,13. Although spin–orbit torques led to domain nucleation in continuous films14 and to stochastic nucleation of skyrmions in magnetic tracks15, no practical means to create individual skyrmions controllably in an integrated device design at a selected position has been reported yet. Here we demonstrate that sub-nanosecond spin–orbit torque pulses can generate single skyrmions at custom-defined positions in a magnetic racetrack deterministically using the same current path as used for the shifting operation. The effect of the DMI implies that no external in-plane magnetic fields are needed for this aim. This implementation exploits a defect, such as a constriction in the magnetic track, that can serve as a skyrmion generator. The concept is applicable to any track geometry, including three-dimensional designs16.

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Figure 1: Simulation of a bit sequence in a racetrack memory created by spin–orbit torque skyrmion generation near a pinning centre.
Figure 2: Experimental set-up and the effect of pulse trains.
Figure 3: Creation of skyrmions at natural defects.
Figure 4: Demonstration of single-skyrmion generation and subsequent motion.

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Acknowledgements

This work was supported by the US Department of Energy, Office of Science, Basic Energy Sciences under Award no. DE-SC0012371. F.B. acknowledges financial support by the German Science Foundation under grant no. BU 3297/1-1.

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Contributions

F.B., B.P., S.E. and G.S.D.B. conceived and designed the experiment. F.B., I.L., M.S., C.M.G. and D.E. prepared and pre-characterized the samples. F.B., I.L., M.S., B.P., P.H., J.G. and L.C. performed the experiments with support by J.V. B.P. and P.H. reconstructed the holographic images. I.L., F.B. and B.K. performed the micromagnetic simulations. F.B. drafted the manuscript. S.E. and G.S.D.B. supervised the project. All the authors discussed the results, the implications and the figures, and commented on the manuscript.

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Correspondence to Felix Büttner.

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Büttner, F., Lemesh, I., Schneider, M. et al. Field-free deterministic ultrafast creation of magnetic skyrmions by spin–orbit torques. Nature Nanotech 12, 1040–1044 (2017). https://doi.org/10.1038/nnano.2017.178

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